FertilityInfo widget to complement CityInfo

Mostly yummy copy/pasta. I could not bear copying over the positioning
code however, so that one is coupled to cityinfo for the time being.

Needs an own widget mostly because it's also supposed to show up when
the island in question has not been settled on yet.
Remove overambitious settlement.fertility

Let's first get this in shape for just islands, and expand from there as
necessary. The cityinfo no longer polls this through settlement, so it's
unused without a good reason to stay.